1. Field of the Invention
The present invention relates to a transmission control system, a subscriber-side communication device and an office-side communication device, and is applicable to a transmission control system, a subscriber-side communication device and an office-side communication device that can normally keep main signal continuity even during CPU restart for software update or the like.
2. Description of the Related Art
An access network called Fiber To The Home (FTTH) using an optical fiber as a transmission channel is becoming widely used for the purpose of providing a high-speed and wideband broadband service to general public home. For provision of a broadband service via the FTTH, an optical access system called Passive Optical Network (PON) is frequently used.
The PON system employs an architecture that connects one Optical Line Terminal (OLT) on the office side and a plurality of Optical Network Units (ONUs) on the subscriber side in a one-to-many relationship by splitting one optical cable using an optical passive element called an optical splitter (e.g. an optical coupler etc.). By sharing an optical fiber and a communication device by a plurality of subscribers, it is possible to provide the FTTH service in an economically effective manner.
The PON system typically uses Wavelength Division Multiplexing (WDM) using different wavelengths for communication from OLT to ONUs and communication from ONUs to OLT. Further, because one fiber is shared by a plurality of ONUs, the interference of signals from the respective ONUs is avoided by using Time Division Multiple Access (TDMA), for example, in the communication from ONUs to OLT.
An access network using the PON system includes Gigabit Ethernet-PON (GE-PON) that uses a Gigabit Ethernet protocol and is standardized as IEEE Std 802.3ah. In the GE-PON, a control function called Multi-Point MAC Control (MPCP) is regulated for access control by the TDMA, and a control frame called a MAC Control frame is exchanged between OLT and ONUs to thereby implement the MPCP control.
In the MPCP, prior to communication between OLT and ONUs, a channel between the OLT and the ONUs is established by a process called Discovery Process. In the Discovery Process, a MAC Control frame for Discovery Process is exchanged between the OLT and the ONUs, thereby establishing a channel between the OLT and the ONUs. Further, for the TDMA control from the ONUs to the OLT, synchronization of time between the OLT and all the ONUs is done in the Discovery Process.
After establishing the channel, each ONU notifies an amount of data requested to be sent (requested transmission amount) to the OLT by using a Report frame, which is one of the MAC Control frame.
Receiving the requested amount of transmission of each ONU notified through the Report frame, the OLT notifies an amount of data allowed to be sent (allowable transmission amount) of each ONU and transmission start time to the ONU by using a Gate frame, which is one of the MAC Control frame.
The ONU sends a data frame to the OLT based on the allowable transmission amount and the transmission start time notified through the Gate frame.
Further, in the IEEE Std 802.3ah, a function related to Operations, Administration and Maintenance (OAM) between OLT and ONUs is regulated, and OAM information is communicated between OLT and ONUs by using a control frame called a Slow Protocol frame. For communication of OAM information between OLT and ONUs, after a channel by the MPCP is established, a channel for OAM information is established by the OAM discovery mechanism. After the channel for OAM information is established, periodical exchange of OAM information is monitored to thereby confirm the normality of the channel for OAM information.
The MPCP discovery process, the exchange of control information through the Gate frame and the Report frame after channel establishment, and the operations, administration and maintenance function which are described above are generally implemented by software processing by a CPU in a communication device that constitutes the PON system (see Japanese Unexamined Patent Publication No. 2009-65575).